1. Field of the Invention
The subject invention relates to fuel pumps for gas turbine engines, and more particularly, to vane pumps which are used in applications that require high operational reliability and a predicted failure mode.
2. Background of the Related Art
Vane pumps are being developed within the aerospace industry as an alternative to traditional gear pumps. An example of a variable displacement vane pump is disclosed in U.S. Pat. No. 5,545,014 to Sundberg et al., the disclosure of which is herein incorporated by reference in its entirety to the extent that it does not conflict with the present disclosure.
Vane pumps traditionally include, among other things, a housing, a cam member and a rotor supported within the housing by axially opposed journal bearings. The housing defines an interior chamber, a fluid inlet and a fluid outlet and the cam member and rotor are disposed within the interior chamber. The cam member has a central bore which defines the circumferential boundary of the internal pumping chamber. Mounted for rotational movement within the central bore of the cam member, is a rotor supported by axial opposed journal bearings. The rotor element has circumferentially spaced apart slots machined therein which support corresponding radially movable vane elements.
Variable displacement vane pumps differ from other vane pumps, such as fixed displacement vane pumps, in that the cam member pivots about a fulcrum aligned with the vertical centerline of the pump, thereby adjusting its position with respect to the rotor. This adjustment allows the relative volumes of the inlet and discharge buckets to be changed and thereby vary the displacement capacity of the pump.
In a single rotation, the vanes of the rotor element of the pump traverse at least four distinct arcuate regions which make up the 360 degree revolution. The first region is the inlet arc segment in which fluid is received into the pumping chamber and over this region the bucket volume increases. The second region is the discharge arc segment in which pressurized fluid is discharged from the pumping chamber and over this region the bucket volume decrease. Lastly, seal arc segments separate the inlet and discharge arc segments and represent the regions through which the bucket volume remains substantially constant.
In operation, fluid at a first pressure is fed into the pumping chamber through the housing inlet, and into the space defined between adjacent vane elements, known as the bucket. In positive displacement vane pumps, as the vane elements rotate within the pumping chamber from the inlet region to the outlet region, the configuration of the cam member causes the vanes to retract within the corresponding slots. This causes the volume defined by the bucket to decrease. Since the amount of fluid received into an inlet bucket is greater than that contained within the corresponding discharge bucket, a fluid volume equivalent in size to the volumetric difference is discharged or displaced through the outlet port at a pressure equal to the downstream pressure which must be overcome.
Typically, pumping pressures and velocities are so high within the pump housing that the use of heavy, high wear resistant materials for the cam member and the vane elements becomes necessary to handle the wear which is caused by these high levels of pressure and velocity.
Prior variable displacement vane pumps are illustrated in U.S. Pat. No. 5,545,014 to Sundberg et al. and U.S. Pat. No. 5,833,438 to Sundberg. U.S. Pat. No. 5,545,014 discloses a durable, single action, variable displacement vane pump capable of undervane pumping and a pressure balancing method. U.S. Pat. No. 5,833,438 to Sundberg teaches a variable displacement vane pump having a durable rotor member with journal ends at each side of a large diameter central vane section and a mechanism for confining the high pressure within the cam member and thereby preventing axial pressure leakage along the length of the rotor member. The disclosure contained within these patent is hereby incorporated by reference in their entirety to the extent it does not conflict with the present disclosure.
The advantages of variable displacement pumps over conventional pumps, namely gear pumps, is that they solve the problem where excess heat generation becomes a crucial impediment to pump performance. Also, a variable displacement vane pump can be used to eliminate certain fuel flow metering components by utilizing the pump as the metering device.
One of the disadvantages associated with vane pump technology is the failure mode. As a result, there is a reluctance to implement this technology in applications, such as high performance aircraft, that require high operational reliability and a predicted failure mode. With a conventional gear pump, the failure mechanism is well known. Typically as the pump degrades, the performance drops off far enough so that eventually one cannot start the engine, thus a safe failure occurs. With a vane pump, however, as the vanes wear away due to contact with the cam surface, the cantilevered load that the pressure puts on each vane can become so high that a catastrophic failure of a vane can occur during pump operation and effectively destroys the whole pumping system without warning. In an applications such as helicopter fuel systems, this type of failure can cause damage to the control system and engine. In order to prevent such an occurrence, the vane pump must be inspected and maintained frequently.
In view of the foregoing, a need exists for an improved vane pump which resembles the failure mode of a gear pump by xe2x80x9ctrackingxe2x80x9d wear of the vanes, and disabling the engine from starting after a certain level of wear is attained.
The subject application is directed to vane pumps for use with gas turbine engines which include a mechanism for altering the failure mode of the pump thereby preventing an operational failure. In a preferred embodiment, the vane pump includes a pump housing, a cam member, a rotor member and a mechanism for communicating a high pressure fluid from the discharge arc region to the inlet arc region so as to prevent pump start-up when a predetermined wear state has been reached.
The pump housing typically includes a cylindrical interior chamber which defines a central axis through which a vertical centerline and a horizontal centerline extend. The cam member is mounted for pivotable movement within the interior chamber of the pump housing about a fulcrum aligned with the vertical centerline of the interior chamber. The cam member has a bore extending therethrough which defines a circumferential surface of a pumping cavity. The circumferential surface of the pumping cavity includes a discharge arc segment, an inlet arc segment and seal arc segments separating the inlet arc segment and the discharge arc segments from one another.
The cylindrical rotor member is mounted for rotational movement within the bore of the cam member about the central axis of the interior chamber. The rotor member has a central body portion with first and second axially opposed end surfaces and a plurality of circumferentially spaced apart radially extending vane slots formed therein. Each vane slot supports a corresponding vane element mounted for radial movement therein. Each of the vane elements have a radially outer tip surface which is adapted for slideably engaging the circumferential surface of the pumping cavity and a radially inner undervane portion which is positioned within each vane slot.
The mechanism for communicating a high pressure fluid from the discharge arc region to the inlet arc region so as to prevent pump start-up activates when the tip surface of each vane element has worn a predetermined amounted with respect to the undervane portion of each vane element.
In a preferred embodiment, the mechanism for communicating a high pressure fluid from the discharge arc region to the inlet arc region when the tip surface of each vane element has worn a predetermined amount includes arcuate channels formed in the first end surface of the body portion of the rotor member. The arcuate channels each extend between each vane slot. It is envisioned that the arcuate channels are spaced from the central axis by a radial distance and the radial distance defines the predetermined amount of wear.
In an alternate embodiment, the means for communicating a high pressure fluid from the discharge arc region to the inlet arc region when the tip surface of each vane element has worn a predetermined amount includes arcuate channels formed in the second end surface of the body portion of the rotor member
It is presently preferred that the predetermined amount of wear is reached when the undervane portion of each vane element at a point in the pumping cavity is positioned radially outward of the arcuate channels formed in the body portion of the rotor. As a result of this relative positioning, fluid is allowed to communicate from the discharge arc segment to the inlet arc segment of the pumping cavity.
The circumferential surface of the pump cavity includes a discharge arc segment of about 150 degrees, a first seal arc segment of about 30 degrees, an inlet arc segment of about 150 degrees and a second seal arc segment of about 30 degrees.
It is further envisioned that first and second axially spaced apart end plates are disposed within the interior chamber of the pump housing. Each end plate has a first surface which is adjacent to the rotor member and forms an axial end portion of the pumping cavity. Each end plate is spaced from the rotor member so as to allow frictionless rotation of the rotor member within the pumping cavity. Preferably the end plates include a mechanism associated with the first surface of each end plate for communicating fluid from the discharge arc segment of the pumping cavity to the undervane portion of each vane element when each vane element passes through the discharge and seal arc segments. Additionally, the first surface of each end plate includes a mechanism for communicating fluid from the inlet arc region of the pumping cavity to the undervane portion of each vane element when each vane element passes through the inlet arc segment as the rotor member rotates about the central axis.
It is presently envisioned that the rotor member further includes a plurality of substantially axial fluid passages formed in the central body portion of the rotor. Each passage is positioned between the plurality of circumferentially spaced apart radial vane slots and provides a path through the rotor body portion for fluid to communicate axially from the pumping cavity to the first and second end plate.
The subject application is also directed to a vane pump which includes, among other things, a pump housing a cam member, a rotor member. The rotor member being substantially cylindrical and mounted for rotational movement within the bore of the cam member about the central axis of the interior chamber. The rotor member includes a central body portion with first and second axially opposed end surfaces and a plurality of circumferentially spaced apart radially extending vane slots formed therein.
It is envisioned that each vane slot supports a corresponding vane element mounted for radial movement therein. Each vane element has a radially outer tip surface adapted for slideably engaging the circumferential surface of the pumping cavity and a radially inner undervane portion within each vane slot. Preferably, the first end surface of the body portion has arcuate channels formed therein which extend between each vane slot. The arcuate channels providing a path for high pressure fluid to leak from the discharge arc segment to the inlet arc segment of the pumping cavity when each vane tip surface has worn such that the undervane portion is positioned radially outward of the arcuate channels.
In a preferred embodiment, the arcuate channels are spaced from the central axis by a radial distance whereby the radial distance defines an amount of allowable vane tip surface wear which can occur before high pressure fluid can leak from the discharge arc segment to the inlet arc segment of the pumping cavity.
The present application is also directed to a vane pump which includes a pump housing, a cam member, a rotor member, a leak path, first and second axially spaced apart end plates. The leak path communicates fluid from the discharge arc region to the inlet arc region when the cam member is in a start-up position and each undervane portion is positioned radially outward of the leak path. It is envisioned that the leak path includes arcuate channels formed in the first end surface of the body portion of the rotor member which extend between each vane slot.
Those skilled in the art will readily appreciate that the inventive aspects of this disclosure can be applied to any type of vane pump, such as fixed or variable displacement vane pumps.